For various applications it is desirable to create a measurement region with a homogeneous magnetic field. The measurement region will generally be positioned within the bore of an electromagnet, although in some applications an array of permanent magnets may be used. A homogeneous magnetic field is desirable in particular in Nuclear Magnetic Resonance (“NMR”) and Magnetic Resonance Imaging (“MRI”) applications.
The magnetic field will generally include a desired field, which ideally should be homogeneous through the measurement region, together with a number of contaminant harmonics that create inhomogeneities. Inhomogeneity in the magnetic field arises for various reasons, including: magnet design, manufacturing deviations in the assembly of the magnet, the presence of ferromagnetic materials in or near the magnets or differences in the magnetic susceptibility of materials used to manufacture the magnet or within the sample volume.
In practice, creation of high quality magnetic field homogeneity is difficult. To improve magnetic field homogeneity, ferromagnetic or electrical shimming of the field may be used.
Ferromagnetic shims are discussed, for example, in EP0272411B1, U.S. Pat. No. 6,313,634 and U.S. Pat. No. 5,047,720. The process of ferromagnetic shimming is generally achieved by one of two methods, both relying on placement of passive shims or tiles of ferromagnetic material in the magnetic field.
First, a number of ferromagnetic ‘tiles’ may be placed at fixed locations to form a pattern of tiles within a magnet bore. The placement of ferromagnetic material within the magnet bore will distort the magnetic field by a known amount. An algorithm can be used to calculate an initial arrangement of tiles to create the required correction harmonics. In general each individual shim tile contributes to each correction harmonic. After the initial placement of tiles, the number and position of the tiles can be adjusted iteratively, based on measurements of the magnetic field. This process is usually iterated a number of times, adding or subtracting shim tiles until the magnetic field is improved to an acceptable degree. Typically there will be several hundred to a thousand or more tiles used. The iterative placement of the tiles can be time consuming.
Second, a predefined shape of ferromagnetic material can be fashioned to correct for a given number of contaminant harmonics.
The above approaches are both complex and time consuming to install, or do not allow adequate ‘tuning’ of the field.
Electrical shimming is also used in some applications. Electrical shimming involves controlled variation of the current in a set number of shimming coils to create a relatively pure harmonic correction. However, this requires further coils and power sources with associated control requirements. It is therefore a more expensive solution than ferromagnetic shimming. Further, the magnitude of correction available by electrical shimming is limited by how much current can be passed through the available volume without unacceptable heat production. Heat production is particularly problematic in NMR magnets where changes in local temperature can affect the performance of the NMR measurement components. In this case a cooling air supply can be employed but total heat production should preferably be restricted to 10 watts or less. This creates a limit to the amount of current that can be introduced into the electric shims. There is no heat generated by ferromagnetic shims in a static background magnetic field.
Reference to any prior art in this specification does not constitute an admission that such prior art forms part of the common general knowledge.
It is an object of the invention to provide improved magnetic field shimming and/or an alternative method of improving magnetic field homogeneity, or at least to provide the public with a useful choice.
It is an alternative object of the invention to provide an improved magnet arrangement for providing a desired magnetic field profile, or at least to provide the public with a useful choice.